Process for improving the stability of base metal thermoelements



United States atet Alois Seidl, Lam, Niederbayern, Germany, assignor toDeutsche Goldund Silber-Scheitleanstalt vormals Roessler, Frankfurt amMain, Germany No Drawing. Application March 16, 1956 Serial No. 571,370

Claims priority, application Germany March 22, 1955 6 Claims. (Cl.148-414} and that as a result thereof their accuracy suffers during eextending use in addition to undergoing possible mechanical damage. Forexample, especially in sulfur containing atmospheres, base metalthermoelectric elements, such as, those of nickel, nickel-chromium oriron construction,

relatively rapidly become brittle and subject to breakage.

Previously, attempts have been made to overcome these difiiculties bythe use of protective tubes which enclose the actual measuring elements.However, the difiiculty in selecting a suitable material for such tubeswith reference to chemical stability, stability against changes intemperature and mechanical strength often provide new problems whichprevented a satisfactory solution.

It is an object of the invention to provide a way to increase thestability of base metal wires or rods such as are used in the form ofthermocouples for electrical measuring instruments so that they can beused for long periods of time Without deleterious attack upon the metalof the wires or rods and the accompanying reduction in accuracy of suchmeasuring instruments.

According to the invention, it was found that this object could beachieved by applying a special coating upon the wires or rods employedfor such measuring instruments. The special coatings according to theinvention are not only stable in the furnace atmospheres usuallyencountered, including those of oxidising, as well as of reducingnature, but also especially those containing gaseous sulfur containingcompounds.

The special coatings according to the invention are produced by applyinga coating of mixed silicates, which preferably is thin, upon thesurfaces of the wires or rods.

The mixed silicate coating according to the invention, contrary to thenormal practices in applying enamel-like coatings, is applied as anessentially aqueous solution of an alkali metal silicate and of a watersoluble compound of a second'metal, which is built into the coatingachieved in addition to the alkali metal. Advantageously, the watersoluble additional metal salt is added directly to the alkali metalsilicate solution applied to the wires or rods in the form of a watersoluble alkali stable salt, for example, the additional metal can be inthe form of an alkali stable complex salt. After application of thecoatings to the surfaces of the wires or rods, such coatings aresubjected to a heat treatment at a temperature above 200 C. in anon-reducing atmosphere. During such heat treatment, oxides formed fromthe base metal migrate into the coating and are taken up thereby in anoticeable amount and this provides for an extraordinarily strong J icebond and coalescence between the coating and the base metal.

Heavy metals primarily come into consideration as the additional metalwhich is to be introduced into the coating as a mixed silicate with thealkali metal. Preferably, multivalent heavy metals are employed whichdeliver ions in aqueous solution having at least two different valences.Such metals, for example, are arsenic, bismuth, chromium, copper, iron,manganese, molybdenum, nickel, lead, antimony and, preferably, cobalt orvanadium. However, other metals which are monovalent, such as, silver orzinc, can also be used. Of course, mixtures of such additional metalscan also be incorporated in the mixed silicate coating according to theinvention.

Preferably, the heat treatment of the coating according to the inventionis eifected after it has been dried. The heat treatment can be a specialheat treatment or such heat treatment can occur when the thermoelementis first put into use. It is only essential that'such heat treatment iseffected in a non-reducing atmosphere, as this is essential for theformation of the oxides of the base metals which are taken up by thecoating and provide the excellent bond between the coating and the basemetal.

The protective coating produced according to the invention can bemaintained extraordinarily thin in com parison to normal enamelcoatings. Expediently, the thickness of a coating according to theinvention is not more than 50g. As a result, the protective coating,which is substantially impervious to gases, is extraordinarily elasticand can be applied to long lengths of wire which can then be wound onrolls without damage to the protective coating. Furthermore, wirescoated according to the invention can be welded together Without removalof the coating for the production of thermocouples Without causing anydifficulties of contact at the weld. Of course,

when the thermocouples are produced by welding precoated wires, thewelded portion can subsequently also receive a protective coatingaccording to the invention.

The coatings according to the invention can be applied to the wires orrods or thermocouples by the usual methods by dipping them in orspraying or painting them with the appropriate solution. It isessential, however, to select the coating thickness sufliciently thinthat drop formation does not occur at the high temperatures at which thethermoelements coated therewith are used. After the silicate containingsolutions are applied they are generally heated for one hour or more attemperatures of 200 C. and higher in a non-reducing atmosphere. The timeand temperature of the heat treatment is so selected with reference tothe base metal coated that a small quantity of oxide of the base metalis built into the silicate coating which is produced.

In the production of the silicate coating solutions according to theinvention a portion of the water can be replaced by essentially inertwater soluble organic solvents, such as, alcohols, aldehydes, ketonesand the like.

Preferably, the alkali metal silicate employed in the production of thecoating solution is one in which the proportion of alkali metal oxide tosilicon dioxide is 113.3 to 3.9.

The following examples will illustrate several modifications accordingto the invention:

Example 1 10 cc. commercial N21 O:SiO -:l:3.9)

7 cc. 8% aqueous ethyl alcohol 5 cc. 1% aqueous solution of Na Co(N0water glass solution (d.=l.27;

After removing the thermocouple from the solution, it was dried in airand then heated in air for 1 hour at 200 C.

Example 2 7 cc. of a 1% aqueous solution of cobalt nitrate .were mixedwith -5 cc. of a 5% aqueous ammoniumnitrate solution and theresultingsolution then slowly stirredinto .10 ccaofa water glass solution of 'thesame compositionasjn Example 1. The Wires which are to be protected,nickel Wires and nickel chromium Wires were spooled oil from rolls andpassed through the solution and thereafter passed through adryingovenmaintained at 200m 250 C. for aperiod-of 15 minutes and then Woundbackon spools.

The following are further examplesof coating solutions according t0 theinvention which can be applied to base metal wires or rods to be usedfor thermocouples or to the thermocouples themselves in a manneranalogous to those describedin Examples 1 and 2:

Example 3 A coating solution was prepared by mixing:

10 cc. of a 0.05 aqueous sodium chromate solution 7 cc. of 0.l% aqueousNa Fe(CN) solution Example 5 A coating solution was prepared by mixing:

107cc. of a water glass solution (d.=1.33) 5 cc. of water 7 cc. of a 1%aqueous K Ni(CN) solution Example 6 A coating solution Was prepared bymixing 10 cc. of a Water glass solution (d.=1.33) with 5 cc. of Water,dissolving 0.1 gram of V 0 therein and adding 5 cc. of a 5% aqueousethanol solution.

Example 7 A coating solution was prepared by mixing:

I claim:

1. A process for increasing the chemical stability of base metal Wiresand rods employed for thermoelectric measurement of temperatures whichcomprises applying a thin coating of an aqueous solution of an alkalimetal sit containing a relatively small quantity of at least one watersoluble heavy metal salt to the surface thereof and subsequently heatingthe coating to a temperature over 200 C. in a no reducing atmosphereuntil oxides formed from the base metal ,base aretaken up bysuchcoating.

2. A process according to claim 1 in which said :heavy metal salt is asalt of a multivalent heavy metal ,WhiCh delivers ions in aqueoussolutions having at least 2 dificrent valences.

3. A process according to claim 1 in which said heavy metal salt is acobalt salt.

4. A process according to claim 1 in which said coating is up to 50thick.

5. A process for increasing the chemical stability of base metalthermocouples which comprises applying .a thin coating of an aqueoussolution of an alkali metal silicate containing a relatively smallquantityvofatgleast one water soluble heavy metal salt tothesurface-thereof and subsequently heating the coating to a temperatureover 200 C. in a non-reducing atmosphere untiloxides formed from thebase metal base are taken up by such coating.

6. A base metal thermocouple, the surfaces of which are protected by acoating produced by the process according to claim 5.

References Cited in the file of this patent UNITED STATES PATENTS2,321,658 Chester June15, 1943

1. A PROCESSS FOR INCREASING THE CHEMICAL STABILITY OF BASE METAL WIRESAND RODS EMPLOYED FOR THERMOELECTRIC MEASUREMENT OF TEMPERATURES WHICHCOMPRISES APPLYING A THIN COATING OF AN AQUEOUS SOLUTION OF AN ALKALIMETAL SILICATE CONTAINING A RELATIVELY SMALL QUANTITY OF AT LEAST ONEWATER SOLUBLE HEAVY METAL SALT TO THE SURFACE THEREOF AND SUBSEQUENTLYHEATING THE COATING TO A TEMPERATURE OVER 200*C. IN A NON-REDUCINGATMOSPHERE UNTIL OXIDES FORMED FROM THE BASE METAL BASE AE TAKEN UP BYSUCH COATING